Multilayered ceramic capacitors and their structures are well known. Several manufacturing steps are currently employed for the production of multilayered ceramic capacitors.
In making a multilayered ceramic capacitor, dielectric ceramic powder is compounded with various ingredients to achieve special electrical properties. This powder is dispersed in solvents and binders to make a slip or slurry. From the slip, a green or unfired dielectric layer is formed usually by doctor blade casting onto a steel belt.
Electrodes are deposited, usually by silk screen printing onto the green sheet. Alternatively, the green sheet may be cast on a release carrier or paper to facilitate green sheet storage and the electrodes subsequently printed on the green sheet. Another green sheet is layered over the printed green sheet and this other green sheet has an electrode pattern printed thereon. A sequence of dielectric-electrode-dielectric layers is continued. The electrode patterns of successive sheets are offset from one another. After the desired number of layers has been reached, the sheets are pressed or laminated and cut into individual capacitors, which are sintered or fired. Then, end terminations are formed on the capacitors to provide a means for physical and electrical connection to the circuit.
In each of the various stages or steps of the process, potential problems exist which can affect the quality of the finished capacitor. The major potential problems exist in the initial steps of the process, particularly achieving a uniform, flat electrode surface, providing a dielectric layer where the average thickness and the minimum thickness approach equality and in the alignment and stacking of the printed sheets.
More specifically, for printing the electrodes, stainless steel screens typically at 325 mesh are used. The ink is normally forced through with a rubber squeegee. The surface that results is not flat and uniform. Holes and asperities in the deposited electrode result in variable dielectric thicknesses between the electrode layers of the finished capacitors. Other prior art printing techniques also do not solve the problems of wavy edges, rough surfaces and non-uniformity throughout the electrical area.
The presence of bumps in the rough electrode surfaces is known to produce undesirable enhancements of the electrical fields between the dielectric layers. In addition, variations in electrode thickness reduce the dielectric thickness at the bumps to produce non-uniformity and perturbations in the electrical fields. The combination of the reduced dielectric thickness and the disturbed fields caused by the bumps leads to electrical insulation weak spots in the capacitor. Prior art electrode coatings normally 150 microinches thick, are non-uniform and can vary 35 to 40% in thickness. When the electrode pattern is printed on the green dielectric sheet, these lumps remain adding to the thickness and non-uniformity. These non-uniformities can result in material stresses in the finished capacitor. Usually the problem is attempted to be solved by pressure lamination to drive the electrode pattern down into the dielectric sheet or vice versa. The pressure exerted on the stack during lamination may add to the stress level.
A typical dielectric is coated at an average thickness of 1200 microinches to form a green sheet. This dielectric thickness may vary by as much as 30% due to lumps or other irregularities. However, holding a green sheet up to the light, one normally sees a variety of thin spots, streaks and other coating non-uniformities. Because of coating irregularities, if a minimum thickness of 800 microinches is desired, the coating may be applied at an average of 1200 microinches to ensure the minimum. Capacitor reliability is a function of this minimum thickness. However, the greater the average coating thickness, the lower the capacitance and the greater the number of layers required to produce the target capacitance. The capacitance of each layer is defined by the following equation: EQU C=KA/T
where C is defined as capacitance, K as the dielectric coefficient, A as the area of the dielectric sandwiched between the two electrode layers and T as the thickness of the dielectric layer. Obviously, dielectric thickness irregularities are highly undesirable due to the impact on total capacitance.
Usually several blank sheets of dielectric form a base layer to give mechanical strength and electrical insulation to the finally formed capacitor. The sheets are printed as they are stacked one on top of the other totalling up to 60 or more layers. It is desirable to align the sheets precisely to produce the proper overlap of electrodes. When the alignment is not precise enough, yield losses can occur. Every other electrode pattern is offset defining "inside foil" and "outside foil" layers which can be connected later. Finally, for strength and electrical insulation, several blank sheets of dielectric are added to the top. The stack is then pressed in a hydraulic press to make a solid laminated stack. Defects, such as debris and line spreading (or blooming) are generated during the handling, stacking and laminating operations. These defects usually manifest themselves as electrical shorts, opens, voltage breakdowns or reliability failures. These defects may not be discovered until after the multilayered ceramic substrate is formed, thereby leading to a defective and non-repairable component. It is believed that these defects are due, at least in part, to the fact that the electrodes (conducting metal pattern or circuitry) are not flush with (do not lie in the same plane as) the surface of the dielectric sheet on which it is printed. As these sheets are stacked the degree of surface irregularity increases from layer to layer increasing the probability of unreliability or voltage breakdown failure.
A precision knife or saw cuts the pressed stack into the tiny individual capacitors. Very careful firing is carried out to burn out the organic binders from the ink and ceramic layers. Too rapid heating causes delamination. Temperatures and heating rates are determined experimentally. Usually, over about 24 hours, the capacitors will be fired to as high as 1400.degree. C. (about 2600.degree. F.) causing the ceramic powder to sinter into a solid ceramic. During this firing, the electrode metal particles also fuse together to form continuous metal foil sheets.
After a careful cooling process, the tiny individual capacitors are placed in a holding fixture. The ends of the capacitor are dipped into other electrode materials, such as silver palladium ink, containing special glass particles called frit. The frit is specially formulated to adhere to the ceramic and to form a structure which keeps the silver-palladium termination from being dissolved in later processing steps. After dipping, a second heat treating step is needed to fire or fuse the end termination ink. When properly done, the fired end termination ink melts and sticks to the exposed electrode foil layers in the body of the capacitor. If not fired hot enough, the end termination will fail to make electrical contact with some of the body electrodes. Heated too rapidly, the capacitor may crack open. At this point, the capacitor is electronically complete and the capacitors are sorted by capacitor value and undergo a wide variety of tests.
The parts are labelled with capacitance value and tolerance. At this stage, they may be used in surface mount technology. Other applications may require that wire leads be attached to the multilayered ceramic capacitor. This use may require the capacitor be dipped in a protective plastic to prevent moisture or corrosion from damaging the part in use.
The process, as currently practiced, suffers from a variety of problems. Irreparable damage to the product can occur at each step. Most importantly, only after firing the final end termination can the part be measured. The problems of each step are hard to track because measurement cannot be made until so late in the process. Defects are thus usually not discovered until after the multilayered ceramic capacitor is formed. The most severe problem of a typical manufacturing process is that between about 25 to 30% of the capacitors do not work at all due to delaminations, electrical shorts or having the wrong capacitance value.
In spite of the multitude of attempts to produce uniform thin electrode coatings and uniform thin dielectric coatings, the prior art problems of bumps, unevenness, line spreading etc. are still prevalent.